Porous metals have found extensive applications in filtration, gas-flow control, catalysis, fuel cells, and batteries. Their open and interconnected microstructure allows facile diffusion of reactants and electrolytes to their relatively large surface areas. Conventionally, porous metals have been grouped into three categories on the basis of their microstructure, these categories being metal sponges or foams, supported aggregated colloidal metals, and micromachined metals. Metal sponges and supported aggregated colloidal metals represent the most important classes of porous metals and have been used for many years in industrial applications. Typical examples of these materials are platinum and palladium catalysts. Metal sponges are composed of an irregular framework of metal, 1 micrometer to 100 micrometers thick, surrounding interconnected pores with sizes in the range 0.5 micrometer to about 160 micrometers. Aggregated colloidal metals are composed of small primary particles of metal, typically spherical and of diameters in the range 2 nanometers to 1 micrometer, which are aggregated into large particles of irregular size and shape. These aggregated particles are often supported by non-metallic materials. The variability in pore sizes of metal sponges and aggregated colloidal metals is typically quite high. The sizes of pores in these materials span the top end of the mesoporous range, and the macroporous range. For the purpose of this application, the term mesoporous refers to pore sizes in the range approximately 13 to 200 .ANG., and macroporous refers to pore sizes greater than about 200 .ANG..
Conventional processes for preparing porous metals include powder sintering and electrochemical deposition. However, as described above these tend to produce metals with a variable pore size, generally in the macroporous range, which may not give a large specific surface area and the variable pore size does not permit its use for size-selective catalysis.
In the drive towards providing porous metals showing improved properties, for use in for example catalysts, batteries, fuel cells, electrochemical capacitors, quantum confinement effect devices, optical devices, sensors, electrosynthesis, electrocatalysis and filtration or chemical separation, to our knowledge no one has yet succeeded in developing an effective process for preparing at least mesoporous metal-based materials of regular structure and uniform pore size, with the attendant advantages in terms of properties which such metal-based materials might be expected to show.
Previously, we have shown that porous ceramic oxide monoliths can be condensed from lyotropic liquid crystalline phase media, whereby the liquid crystalline phase topology directs the synthesis of the material into a corresponding topology showing structural regularity and uniformity of pore size. However, it was not expected that this templating mechanism could be used to synthesise porous metal-based materials.